The present invention relates to optical sensors and, more particularly, to optical sensors using etalons for remote sensing in extreme environments.
There are numerous vital sensing scenarios in commercial and defense sectors where the environment is extremely hazardous. Specifically, the hazards can be for instance due to extreme temperatures, extreme pressures, highly corrosive chemical content (liquids, gases, particulates), nuclear radiation, biological agents, and high Gravitational (G) forces. Realizing a sensor for such hazardous environments remains to be a tremendous engineering challenge. One specific application is fossil fuel fired power plants where temperatures in combustors and turbines typically have temperatures and pressures exceeding 1000° C. and 50 Atmospheres (atm). Future clean design zero emission power systems are expected to operate at even high temperatures and pressures, e.g., >2000° C. and >400 atm [J. H. Ausubel, “Big Green Energy Machines,” The Industrial Physicist, ALP, pp. 20-24, October/November, 2004.] In addition, coal and gas fired power systems produce chemically hazardous environments with chemical constituents and mixtures containing for example carbon monoxide, carbon dioxide, nitrogen, oxygen, sulphur, sodium, and sulphuric acid. Over the years, engineers have worked very hard in developing electrical high temperature sensors (e.g., thermo-couples using platinum and rodium), but these have shown limited life-times due to the wear and tear and corrosion suffered in power plants [R. E. Bentley, “Thermocouple materials and their properties,” Chap. 2 in Theory and Practice of Thermoelectric Thermometry Handbook of Temperature Measurement, Vol. 3, pp. 25-81, Springer-Verlag Singapore, 1998].
Researchers have turned to optics for providing a robust high temperature sensing solution in these hazardous environments. The focus of these researchers has been mainly directed in two themes. The first theme involves using the optical fiber as the light delivery and reception mechanism and the temperature sensing mechanism. Specifically, a Fiber Bragg Grating (FBG) present within the core of the single mode fiber (SMF) acts as a temperature sensor. Here, a broadband light source is fed to the sensor and the spectral shift of the FBG reflected light is used to determine the temperature value. Today, commercial FBG sensors are written using Ultra-Violet (UV) exposure in silica fibers. Such FBG sensors are typically limited to under 600° C. because of the instability of the FBG structure at higher temperatures [B. Lee, “Review of the present status of optical fiber sensors,” Optical Fiber Technology, Vol. 9, pp. 57-79, 2003]. Recent studies using FBGs in silica fibers has shown promise up-to 1000° C. [M. Winz, K. Stump, T. K. Plant, “High temperature stable fiber Bragg gratings, “Optical Fiber Sensors (OFS) Conf. Digest, pp. 195 198, 2002; D. Grobnic, C. W. Smelser, S. J. Mihailov, R. B. Walker,” Isothermal behavior of fiber Bragg gratings made with ultrafast radiation at temperatures above 1000 C,” European Conf. Optical Communications (ECOC), Proc. Vol. 2, pp. 130-131, Stockholm, Sep. 7, 2004]. To practically reach the higher temperatures (e.g., 1600° C.) for fossil fuel applications, single crystal Sapphire fiber has been used for Fabry-Perot cavity and FBG formation [H. Xiao, W. Zhao, R. Lockhart, J. Wang, A. Wang, “Absolute Sapphire optical fiber sensor for high temperature applications,” SPIE Proc. Vol. 3201, pp. 36-42, 1998; D. Grobnic, S. J. Mihailov, C. W. Smelser, H. Ding, “Ultra high temperature FBG sensor made in Sapphire fiber using Isothermal using femtosecond laser radiation,” European Conf. Optical Communications (ECOC), Proc. Vol. 2, pp. 128-129, Stockholm, Sep. 7, 2004]. The single crystal Sapphire fiber FBG has a very large diameter (e.g., 150 microns) that introduces multi-mode light propagation noise that limits sensor performance. An alternate approach [see Y. Zhang, G. R. Pickrell, B. Qi, A. S.-Jazi, A. Wang, “Single-crystal sapphire-based optical high temperature sensor for harsh environments,” Opt. Eng., 43, 157-164, 2004] described replaced the Sapphire fiber frontend sensing element with a complex assembly of individual components that include a Sapphire bulk crystal that forms a temperature dependent birefringent Fabry-Perot cavity, a single crystal cubic zirconia light reflecting prism, a Glan-Thompson polarizer, a single crystal Sapphire assembly tube, a fiber collimation lens, a ceramic extension tube, and seven 200 micron diameter multimode optical fibers. Hence this described sensor frontend sensing element not only has low optical efficiency and high noise generation issues due to its multi-mode versus SMF design, the sensor frontend is limited by the lowest high temperature performance of a given component in the assembly and not just by the Sapphire crystal and zirconia high temperature ability. Add to these issues, the polarization and component alignment sensitivity of the entire frontend sensor assembly and the Fabry-Perot cavity spectral notch/peak shape spoiling due to varying cavity material parameters. In particular, the Sapphire Crystal is highly birefringent and hence polarization direction and optical alignment issues become critical.
An improved packaged design of this probe using many alignment tubes (e.g., tubes made of Sapphire, alumina, stainless steel) was shown in Z. Huang. G. R. Pickrell, J. Xu, Y. Wang, Y. Zhang A. Wang, “Sapphire temperature sensor coal gasifier field test,” SPIE. Proc. Vol. 5590, p. 27-36, 2004. Here the fiber collimator lens for light collimation and the bulk polarizer (used in Y. Zhang, G. R. Pickrell, B. Qi, A. S.-Jazi, A. Wang, “Single-crystal sapphire-based optical high temperature sensor for harsh environments,” Opt. Eng., 43, 157-164, 2004) are interfaced with a commercial Conax, Buffalo multi-fiber cable with seven fibers; one central fiber for light delivery and six fibers surrounding the central fiber for light detection. All fibers have 200 micron diameters and hence are multi-mode fibers (MMF). Hence this temperature sensor design is again limited by the spectral spoiling plus other key effects when using very broadband light with MMFs. Specifically, light exiting a MMF with the collimation lens has poor collimation as it travels a free-space path to strike the sensing crystal. In effect, a wide angular spread optical beam strikes the Sapphire crystal acting as a Fabry-Perot etalon. The fact that broadband light is used further multiplies the spatial beam spoiling effect at the sensing crystal site. This all leads to additional coupling problems for the receive light to be picked up by the six MMFs engaged with the single fixed collimation lens. Recall that the best Fabry-Perot effect is obtained when incident light is highly collimated; meaning it has high spatial coherence. Another problem plaguing this design is that any unwanted mechanical motion of any of the mechanics and optics along the relatively long (e.g., 1 m) freespace optical processing path from seven fiber-port to Sapphire crystal cannot be countered as all optics are fixed during operations. Hence, this probe can suffer catastrophic light targeting and receive coupling failure causing in-operation of the sensor. Although this design used two sets of manual adjustment mechanical screws each for 6-dimension motion control of the polarizer and collimator lens, this manual alignment is only temporary during the packaging stage and not during sensing operations. Another point to note is that the tube paths contain air undergoing extreme temperature gradients and pressure changes; in effect, air turbulence that can further spatially spoil the light beam that strikes the crystal and also for receive light processing. Thus, this mentioned design is not a robust sensor probe design when using freespace optics and fiber-optics.
Others such as Conax Buffalo Corp. U.S. Pat. No. 4,794,619, Dec. 27, 1988 have eliminated the freespace light path and replaced it with a MMF made of Sapphire that is later connected to a silica MMF. The large Numerical Aperture (NA) Sapphire fiber captures the Broadband optical energy from an emissive radiative hot source in close proximity to the Sapphire fiber tip. Here the detected optical energy is measured over two broad optical bands centered at two different wavelengths, e.g., 0.5 to 1 microns and 1 to 1.5 microns. Then the ratio of optical power over these two bands is used to calculate the temperature based on prior 2-band power ratio vs. temperature calibration data. This two wavelength band power ratio method was described earlier in M. Gottlieb, et al., U.S. Pat. No. 4,362,057, Dec. 7, 1982. The main point is that this 2-wavelength power ratio is unique over a given temperature range. Using freespace optical infrared energy capture via a lens, a commercial product from Omega Model iR2 is available as a temperature sensor that uses this dual-band optical power ratio method to deduce the temperature. Others (e.g., Luna Innovations, V A and Y. Zhu, Z. Huang, M. Han, F. Shen, G. Pickrell, A. Wang, “Fiber-optic high temperature thermometer using sapphire fiber,” SPIE Proc. Vol. 5590, pp. 19-26, 2004.) have used the Sapphire MMF in contact with a high temperature handling optical crystal (e.g., Sapphire) to realize a temperature sensor, but again the limitations due to the use of the MMF are inherent to the design.
It has long been recognized that SiC is an excellent high temperature material for fabricating electronics, optics, and optoelectronics. For example, engineers have used SiC substrates to construct gas sensors [A. Arbab, A. Spetz and I. Lundstrom, “Gas sensors for high temperature operation based on metal oxide silicon carbide (MOSiC) devices,” Sensors and Actuators B, Vol. 15-16, pp. 19-23, 1993]. Prior works include using thin films of SiC grown on substrates such as Sapphire and Silicon to act as Fabry Perot Etalons to form high temperature fiber-optic sensors [G. Beheim, “Fibre-optic thermometer using semiconductor-etalon sensor,” Electronics Letters, vol. 22, p. 238, 239, Feb. 27, 1986; L. Cheng, A. J. Steckl, J. Scofield, “SiC thin film Fabry-Perot interferometer for fiber-optic temperature sensor,” IEEE Tran. Electron Devices, Vol. 50, No. 10, pp. 2159-2164, October 2003; L. Cheng, A. J. Steckl, J. Scofield, “Effect of trimethylsilane flow rate on the growth of SiC thin-films for fiber-optic temperature sensors,” Journal of Microelectromechanical Systems, Volume: 12, Issue: 6, Pages: 797-803, December 2003]. Although SiC thin films on high temperature substrates such as Sapphire can operate at high temperatures, the SiC and Sapphire interface have different material properties such as thermal coefficient of expansion and refractive indexes. In particular, high temperature gradients and fast temperature/pressure temporal effects can cause stress fields at the SiC thin film-Sapphire interface causing deterioration of optical properties (e.g., interface reflectivity) required to form a quality Fabry-Perot etalon needed for sensing based on SiC film refractive index change. Note that these previous works also had a limitation on the measured unambiguous sensing (e.g., temperature) range dictated only by the SiC thin film etalon design, i.e., film thickness and reflective interface refractive indices/reflectivities. Thus maker a thinner SiC film would provide smaller optical path length changes due to temperature and hence increase the unambiguous temperature range. But making a thinner SiC film makes the sensor less sensitive and more fragile to pressure. Hence, a dilemma exists. In addition, temperature change is preferably estimated based on tracking optical spectrum minima shifts using precision optical spectrum analysis optics, making precise temperature estimation a challenge dependent on the precision (wavelength resolution) of the optical spectrum analysis hardware. In addition, better temperature detection sensitivity is achieved using thicker films, but thicker etalon gives narrower spacing between adjacent spectral minima. Thicker films are harder to grow with uniform thicknesses and then one requires higher resolution for the optical spectrum analysis optics. Hence there exists a dilemma where a thick film is desired for better sensing resolution but it requires a better precision optical spectrum analyzer (OSA) and of course thicker thin film SiC etalons are harder to make optically flat. Finally, all to these issues the Fabry-Perot cavity spectral notch/peak shape spoiling due to varying cavity material parameters that in-turn leads to deterioration in sensing resolution.
Material scientists have also described non-contact laser assisted ways to sense the temperature of optical chips under fabrication. Here, both the chip refractive index change due to temperature and thermal expansion effect have been used to create the optical interference that has been monitored by the traditional Fabry-Perot etalon fringe counting method to deduce temperature. These methods are not effective to form a real-time temperature sensor as these prior-art methods require the knowledge of the initial temperature when fringe counting begins. For industrial power plant applications, such a prior knowledge is not possible, while for laboratory material growth and characterization, this a prior knowledge is possible. As shown later in this application, our described sensor designs solve this problem and no longer need the initial temperature data as real-time fringe counting is not used. Prior works in this general laser-based materials characterization field include: F. C. Nix & D. MacNair, “An interferometric dilatometer with photographic recording,” AIP Rev. of Scientific Instruments (RSI) Journal, Vol. 12, February 1941; V. D. Hacman, “Optische Messung der substrat-temperatur in der Vakuumaufdampftechnik,” Optik, Vol. 28, p 115, 1968; R. Bond, S. Dzioba, H. Naguib, J. Vacuum Science & Tech., 18(2), March 1981; K. L. Saenger, J. Applied Physics, 63(8), Apr. 15, 1988; V. Donnelly & J. McCaulley, J. Vacuum Science & Tech., A 8(1), January/February 1990; K. L. Saenger & J. Gupta, Applied Optics, 30(10), Apr. 1, 1991; K. L. Saenger, F. Tong, J. Logan, W. Holber, Rev. of Scientific Instruments (RSI) Journal, Vol. 63, No. 8, August 1992; V. Donnelly, J. Vacuum Science & Tech., A 11(5), September/October 1993; J. McCaulley, V. Donnelly, M. Vernon, I. Taha, AIP Physics Rev. B, Vol. 49, No. 11, 15 Mar. 1994; M. Lang, G. Donohoe, S. Zaidi, S. Brueck, Optical Engg., Vol. 33, No. 10, October 1994; F. Xue, X. Yangang, C. Yuanjie, M. Xiufang, S. Yuanhua, SPIE Proc. Vol. 3558, p. 87, 1998.